Bioshock trailer=Vsync issues?

Anyone else have lots of screen tearing during 720p playback of the new gameplay trailer on marketplace?

I know its a work in progress but its a bit worrying considering how other games have been released with this problem unsolved.

Can anyone explain to me exactly what causes screen tearing?

i've noticed that in some gameplay videos, and demos, but have yet to see it in any final releases of games.ibanezdropd